Current EPA-compliant water-borne coatings such as those in which the volatile component contains less than 20% organic solvent are very much in demand, especially for metal decorating and can lining applications.
Although excellent performance has been obtained for many years from systems based on organic solvent-borne, solid, epoxy resin type compositions cured with aminoplast or phenolic resins, these coating systems contain about 60-70% organic solvent, and therefore cannot comply with existing air pollution regulations without recourse to costly incineration of the solvent driven off during the baking step.
Currently, two approaches have been taken to develop a water-borne epoxy resin baking system for use in baked coatings such as can linings: (1) the so-called "high acid number" epoxy resin ester vehicle and (2) emulsions of solid epoxy resins. Both types of compositions are blended with a suitable curing agent, such as the amino resins, to achieve crosslinking during a typical bake schedule of 10 minutes at 400.degree. F. The epoxy ester approach produces good roller coating application properties, i.e. good rheology; however, this ester coating falls short in resistance to food products, and the storage stability of these epoxy types is only fair. On the other hand, the solid epoxy resin emulsion systems give good cured film properties, but have poor application properties on high speed roller coaters.
It would be very desirable to combine the best properties of each system into a single system.
A water-borne system which combines these advantages into a single system is described in copending patent application Ser. No. 782,208, filed Mar. 28, 1977. Although this water-borne system does offer significant improvement over the epoxy resin ester systems and solid resin emulsion systems, these systems have certain shortcomings. Specifically, the compositions described in Ser. No. 782,208 require the use of low molecular weight epoxy resins, i.e. normally liquid resins. Also, the preparation of these compositions require a three-step process.
It has now been found that water-borne baking systems having improved physical properties can be made with solid as well as liquid epoxy resins in a simpler two-step process.